An engine may include a compressor to increase engine power. The compressor may be part of a turbocharger or a super charger. At low engine loads, the compressor may rotate at low engine speeds and provide little boost. As engine load increases, compressor speed may increase to provide increased boost to the engine. However, since the compressor has inertia, it may take longer than is desired for the compressor to reach a speed where it provides a desired level of boost. The delay period between when increased engine power is commanded and when boost pressure reaches a value to meet the increased engine power may be referred to as compressor lag.
One way to reduce compressor lag from a turbocharger or super charger is to install an electrically driven compressor in series with the supercharger or turbocharger. The electrically driven compressor may reduce compressor lag time because the electrically driven compressor may have a shorter response time. Nevertheless, the electrically driven compressor also has to overcome inertia of its compressor and rotating components to meet a desired boost pressure. At least one set of driving conditions expose limitations of an electrically driven compressor. Specifically, during conditions where durations between requested power being high and requested power being low are short, the electric compressor may not be able to accelerate the compressor from a low speed to a higher speed to meet boost requirements. Therefore, a driver of the vehicle may experience some delay in the production of engine power. For example, during a rock crawl mode or traveling off road, a vehicle driver may make short requests for large amounts of engine power to move over rocks in the vehicle's path. As soon as the rock or obstacle is overcome, the vehicle driver may reduce the engine torque demand so that vehicle body motion and vehicle acceleration may be controlled. The vehicle driver may have to overcome several obstacles that are closely spaced resulting in a series of large engine torque demands that are each followed by large engine torque reduction. During such conditions, the electrically driven compressor may not deliver boost as fast as is desired so that the driver may notice some hesitation by the engine to provide the requested torque. Therefore, it would be desirable to provide a timely way of meeting the driver demanded torque when time between large torque demands and small torque demands is short.
The inventors herein have recognized the above-mentioned issues and have developed an engine operating method, comprising: in a first mode, rotating an electrically driven compressor at a base speed while an engine rotates at idle speed and driver demand torque is zero; and in a second mode, rotating the electrically driven compressor at the base speed plus an offset speed while the engine rotates at idle speed and driver demand torque is zero.
By rotating an electrically driven compressor at a base speed plus an offset speed, pressurized air may be made available to the engine even during conditions where a vehicle driver increases and decreases a driver demand torque in a short period of time. The speed offset increases boost pressure so that if the vehicle driver requests torque quickly after releasing the accelerator pedal, a large amount of air may be made available to the engine so that engine torque may be quickly increased.
In other examples, a decreasing electric compressor speed command or request may be low pass filtered so that it takes an increased amount of time for speed of the electric compressor to reach base compressor speed for engine idle conditions. Thus, it may take longer for the compressor speed to be reduced from a higher speed to base speed so that if the driver requests additional torque in a short period of time, electric compressor speed is at a higher level than base electric compressor speed. In this way, additional air may be provided to the engine as compared to if the electric compressor speed was quickly reduced to base speed after a driver released the accelerator pedal.
The present description may provide several advantages. In particular, the approach may provide a higher level of boost at engine idle conditions as compared to if the electric compressor rotated at a speed for base engine idle conditions. Further, the approach may reduce an amount of time for an engine to provide a desired amount of torque. Further still, the approach may be incorporated into a transient power performance mode that may be entered automatically or via human driver input.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.